Red-emitting phosphors based on complex fluoride materials activated by Mn4+, such as those described in U.S. Pat. No. 7,358,542, U.S. Pat. No. 7,497,973, and U.S. Pat. No. 7,648,649, can be utilized in combination with yellow/green emitting phosphors such as YAG:Ce or other garnet compositions to achieve warm white light (CCTs<5000 K on the blackbody locus, color rendering index (CRI)>80) from a blue LED, equivalent to that produced by current fluorescent, incandescent and halogen lamps. These materials absorb blue light strongly and efficiently emit between about 610-635 nm with little deep red/NIR emission. Therefore, luminous efficacy is maximized compared to red phosphors that have significant emission in the deeper red where eye sensitivity is poor. Quantum efficiency can exceed 85% under blue (440-460 nm) excitation.
While the efficacy and CRI of lighting systems using Mn4+ doped fluoride hosts can be quite high, one potential limitation is their susceptibility to degradation under high temperature and humidity (HTHH) conditions and high light fluxes. US 2014/0264418 describes processes that can increase color stability of Mn4+ doped complex fluoride materials containing up to 1.5 wt % manganese. However, improved color stability and other properties important for use with LEDs is desirable for materials containing higher levels of manganese.